1. Field of the Invention
The present invention relates to radio receivers for direct sequence spread spectrum communications, and more particularly, to a radio system that conserves electrical power by discontinuing radio reception upon detection of an invalid embedded address within a transmitted message packet.
2. Description of Related Art
Spread spectrum modulation techniques are increasingly desirable for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency (RF) carrier is modulated by a digital code sequence having a bit rate much higher than that of the information signal. These systems are known as "direct sequence" modulation systems. One type of a direct sequence spread spectrum system includes the RF carrier modulated by two data streams in quadrature with each one having one phase when the data stream code sequence represents a data "one" and 180.degree. phase shift when the data stream code sequence represents a data "zero." This type of modulation is commonly referred to as Quadrature Phase Shift Key (QPSK) modulation.
It is also known to use a plurality of spread spectrum radio receivers that are coupled together in a wireless local area network (LAN). A central host processing unit could send information to and receive information from any one of the plurality of remotely disposed receivers. In such a wireless LAN, the remote receivers may comprise portable units that operate within a defined environment to report information back to the central host processing unit. Each of the remote receivers would communicate with the host processing unit using the same RF carrier frequency and digital code sequence. It should be apparent that such wireless LAN systems offer increased flexibility over hardwired systems by enabling operators of the remote receivers substantial freedom of movement through the environment.
The individual radio receivers amplify and filter an RF signal transmitted from the host processing unit to remove the RF carrier and provide a digital information signal that has been modulated by the digital code sequence. The receiver then "de-spreads" the digital signal by use of a digital match filter that is correlated with the digital code sequence to remove the modulation and recover the digital information. Discrete digital bits of the de-spread digital information are then assembled into packets having a predefined format that can be processed subsequently by use of conventional data processing logic systems, such as a microprocessor, digital signal processor, and the like.
The predefined message packet format typically includes a header portion followed by associated data. The header portion determines the routing of the message by identifying which one of the remote receivers is the intended recipient of the message, and may also include various other predefined data fields, such as message length, error correction codes, etc. Since each individual receiver does not know when to expect a message from the host processing unit, the receivers must constantly monitor all transmitted signals to determine whether a particular message is intended for them. Transmitted data will normally follow the header portion in the message packet.
A drawback with such wireless systems is the limited power source of the remote receivers. The digital signal processing circuitry that performs the de-spreading of the received signals draws a significant amount of electrical power. To maximize flexibility and freedom of movement, the remote receivers include a rechargeable battery system. When not in use, the remote receivers could be plugged into a recharging station that restores the battery system to a fully charged state. Nevertheless, such battery systems necessarily increase the weight and bulk of the remote radio receivers, as a heavier battery system would provide greater storage capacity and longer operational life. Radio system designers must trade off weight of the receiver against its operational life, and have long sought ways to reduce the power requirements of a remote receiver in order to further extend the operational life without having to increase the battery system capacity.
Thus, it would be desirable to provide a remote radio receiver for direct sequence spread spectrum communication system that draws reduced electrical current in order to maximize operational life between charging cycles. More particularly, it would be desirable to provide a remote radio receiver capable of recognizing when a received signal is intended for another receiver and discontinuing radio reception and associated digital signal processing during that discrete period of time.